Shulchan Arukh, Even HaEzer 19/שולחן ערוך·אבן העזר י״ט

1One who is held to be a kinsman [in regards to illicit relations], we judge on this presumption in spite of the fact that there is no clear proof that he is a relative, and we lash [him] and strangle [him] and stone [him] according to this presumption.

2A man and a woman that came from a foreign country, she says: this is my husband, and he says: this is my wife, if it becomes known in that city for thirty days that she is his wife, they kill because of her.